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Diffstat (limited to 'vendor/crossbeam-epoch/src/guard.rs')
| -rw-r--r-- | vendor/crossbeam-epoch/src/guard.rs | 523 |
1 files changed, 0 insertions, 523 deletions
diff --git a/vendor/crossbeam-epoch/src/guard.rs b/vendor/crossbeam-epoch/src/guard.rs deleted file mode 100644 index 5fe3380..0000000 --- a/vendor/crossbeam-epoch/src/guard.rs +++ /dev/null @@ -1,523 +0,0 @@ -use core::fmt; -use core::mem; - -use crate::atomic::Shared; -use crate::collector::Collector; -use crate::deferred::Deferred; -use crate::internal::Local; - -/// A guard that keeps the current thread pinned. -/// -/// # Pinning -/// -/// The current thread is pinned by calling [`pin`], which returns a new guard: -/// -/// ``` -/// use crossbeam_epoch as epoch; -/// -/// // It is often convenient to prefix a call to `pin` with a `&` in order to create a reference. -/// // This is not really necessary, but makes passing references to the guard a bit easier. -/// let guard = &epoch::pin(); -/// ``` -/// -/// When a guard gets dropped, the current thread is automatically unpinned. -/// -/// # Pointers on the stack -/// -/// Having a guard allows us to create pointers on the stack to heap-allocated objects. -/// For example: -/// -/// ``` -/// use crossbeam_epoch::{self as epoch, Atomic}; -/// use std::sync::atomic::Ordering::SeqCst; -/// -/// // Create a heap-allocated number. -/// let a = Atomic::new(777); -/// -/// // Pin the current thread. -/// let guard = &epoch::pin(); -/// -/// // Load the heap-allocated object and create pointer `p` on the stack. -/// let p = a.load(SeqCst, guard); -/// -/// // Dereference the pointer and print the value: -/// if let Some(num) = unsafe { p.as_ref() } { -/// println!("The number is {}.", num); -/// } -/// # unsafe { drop(a.into_owned()); } // avoid leak -/// ``` -/// -/// # Multiple guards -/// -/// Pinning is reentrant and it is perfectly legal to create multiple guards. In that case, the -/// thread will actually be pinned only when the first guard is created and unpinned when the last -/// one is dropped: -/// -/// ``` -/// use crossbeam_epoch as epoch; -/// -/// let guard1 = epoch::pin(); -/// let guard2 = epoch::pin(); -/// assert!(epoch::is_pinned()); -/// drop(guard1); -/// assert!(epoch::is_pinned()); -/// drop(guard2); -/// assert!(!epoch::is_pinned()); -/// ``` -/// -/// [`pin`]: super::pin -pub struct Guard { - pub(crate) local: *const Local, -} - -impl Guard { - /// Stores a function so that it can be executed at some point after all currently pinned - /// threads get unpinned. - /// - /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache - /// becomes full, some functions are moved into the global cache. At the same time, some - /// functions from both local and global caches may get executed in order to incrementally - /// clean up the caches as they fill up. - /// - /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it - /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might - /// never run, but the epoch-based garbage collection will make an effort to execute it - /// reasonably soon. - /// - /// If this method is called from an [`unprotected`] guard, the function will simply be - /// executed immediately. - pub fn defer<F, R>(&self, f: F) - where - F: FnOnce() -> R, - F: Send + 'static, - { - unsafe { - self.defer_unchecked(f); - } - } - - /// Stores a function so that it can be executed at some point after all currently pinned - /// threads get unpinned. - /// - /// This method first stores `f` into the thread-local (or handle-local) cache. If this cache - /// becomes full, some functions are moved into the global cache. At the same time, some - /// functions from both local and global caches may get executed in order to incrementally - /// clean up the caches as they fill up. - /// - /// There is no guarantee when exactly `f` will be executed. The only guarantee is that it - /// won't be executed until all currently pinned threads get unpinned. In theory, `f` might - /// never run, but the epoch-based garbage collection will make an effort to execute it - /// reasonably soon. - /// - /// If this method is called from an [`unprotected`] guard, the function will simply be - /// executed immediately. - /// - /// # Safety - /// - /// The given function must not hold reference onto the stack. It is highly recommended that - /// the passed function is **always** marked with `move` in order to prevent accidental - /// borrows. - /// - /// ``` - /// use crossbeam_epoch as epoch; - /// - /// let guard = &epoch::pin(); - /// let message = "Hello!"; - /// unsafe { - /// // ALWAYS use `move` when sending a closure into `defer_unchecked`. - /// guard.defer_unchecked(move || { - /// println!("{}", message); - /// }); - /// } - /// ``` - /// - /// Apart from that, keep in mind that another thread may execute `f`, so anything accessed by - /// the closure must be `Send`. - /// - /// We intentionally didn't require `F: Send`, because Rust's type systems usually cannot prove - /// `F: Send` for typical use cases. For example, consider the following code snippet, which - /// exemplifies the typical use case of deferring the deallocation of a shared reference: - /// - /// ```ignore - /// let shared = Owned::new(7i32).into_shared(guard); - /// guard.defer_unchecked(move || shared.into_owned()); // `Shared` is not `Send`! - /// ``` - /// - /// While `Shared` is not `Send`, it's safe for another thread to call the deferred function, - /// because it's called only after the grace period and `shared` is no longer shared with other - /// threads. But we don't expect type systems to prove this. - /// - /// # Examples - /// - /// When a heap-allocated object in a data structure becomes unreachable, it has to be - /// deallocated. However, the current thread and other threads may be still holding references - /// on the stack to that same object. Therefore it cannot be deallocated before those references - /// get dropped. This method can defer deallocation until all those threads get unpinned and - /// consequently drop all their references on the stack. - /// - /// ``` - /// use crossbeam_epoch::{self as epoch, Atomic, Owned}; - /// use std::sync::atomic::Ordering::SeqCst; - /// - /// let a = Atomic::new("foo"); - /// - /// // Now suppose that `a` is shared among multiple threads and concurrently - /// // accessed and modified... - /// - /// // Pin the current thread. - /// let guard = &epoch::pin(); - /// - /// // Steal the object currently stored in `a` and swap it with another one. - /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard); - /// - /// if !p.is_null() { - /// // The object `p` is pointing to is now unreachable. - /// // Defer its deallocation until all currently pinned threads get unpinned. - /// unsafe { - /// // ALWAYS use `move` when sending a closure into `defer_unchecked`. - /// guard.defer_unchecked(move || { - /// println!("{} is now being deallocated.", p.deref()); - /// // Now we have unique access to the object pointed to by `p` and can turn it - /// // into an `Owned`. Dropping the `Owned` will deallocate the object. - /// drop(p.into_owned()); - /// }); - /// } - /// } - /// # unsafe { drop(a.into_owned()); } // avoid leak - /// ``` - pub unsafe fn defer_unchecked<F, R>(&self, f: F) - where - F: FnOnce() -> R, - { - if let Some(local) = self.local.as_ref() { - local.defer(Deferred::new(move || drop(f())), self); - } else { - drop(f()); - } - } - - /// Stores a destructor for an object so that it can be deallocated and dropped at some point - /// after all currently pinned threads get unpinned. - /// - /// This method first stores the destructor into the thread-local (or handle-local) cache. If - /// this cache becomes full, some destructors are moved into the global cache. At the same - /// time, some destructors from both local and global caches may get executed in order to - /// incrementally clean up the caches as they fill up. - /// - /// There is no guarantee when exactly the destructor will be executed. The only guarantee is - /// that it won't be executed until all currently pinned threads get unpinned. In theory, the - /// destructor might never run, but the epoch-based garbage collection will make an effort to - /// execute it reasonably soon. - /// - /// If this method is called from an [`unprotected`] guard, the destructor will simply be - /// executed immediately. - /// - /// # Safety - /// - /// The object must not be reachable by other threads anymore, otherwise it might be still in - /// use when the destructor runs. - /// - /// Apart from that, keep in mind that another thread may execute the destructor, so the object - /// must be sendable to other threads. - /// - /// We intentionally didn't require `T: Send`, because Rust's type systems usually cannot prove - /// `T: Send` for typical use cases. For example, consider the following code snippet, which - /// exemplifies the typical use case of deferring the deallocation of a shared reference: - /// - /// ```ignore - /// let shared = Owned::new(7i32).into_shared(guard); - /// guard.defer_destroy(shared); // `Shared` is not `Send`! - /// ``` - /// - /// While `Shared` is not `Send`, it's safe for another thread to call the destructor, because - /// it's called only after the grace period and `shared` is no longer shared with other - /// threads. But we don't expect type systems to prove this. - /// - /// # Examples - /// - /// When a heap-allocated object in a data structure becomes unreachable, it has to be - /// deallocated. However, the current thread and other threads may be still holding references - /// on the stack to that same object. Therefore it cannot be deallocated before those references - /// get dropped. This method can defer deallocation until all those threads get unpinned and - /// consequently drop all their references on the stack. - /// - /// ``` - /// use crossbeam_epoch::{self as epoch, Atomic, Owned}; - /// use std::sync::atomic::Ordering::SeqCst; - /// - /// let a = Atomic::new("foo"); - /// - /// // Now suppose that `a` is shared among multiple threads and concurrently - /// // accessed and modified... - /// - /// // Pin the current thread. - /// let guard = &epoch::pin(); - /// - /// // Steal the object currently stored in `a` and swap it with another one. - /// let p = a.swap(Owned::new("bar").into_shared(guard), SeqCst, guard); - /// - /// if !p.is_null() { - /// // The object `p` is pointing to is now unreachable. - /// // Defer its deallocation until all currently pinned threads get unpinned. - /// unsafe { - /// guard.defer_destroy(p); - /// } - /// } - /// # unsafe { drop(a.into_owned()); } // avoid leak - /// ``` - pub unsafe fn defer_destroy<T>(&self, ptr: Shared<'_, T>) { - self.defer_unchecked(move || ptr.into_owned()); - } - - /// Clears up the thread-local cache of deferred functions by executing them or moving into the - /// global cache. - /// - /// Call this method after deferring execution of a function if you want to get it executed as - /// soon as possible. Flushing will make sure it is residing in in the global cache, so that - /// any thread has a chance of taking the function and executing it. - /// - /// If this method is called from an [`unprotected`] guard, it is a no-op (nothing happens). - /// - /// # Examples - /// - /// ``` - /// use crossbeam_epoch as epoch; - /// - /// let guard = &epoch::pin(); - /// guard.defer(move || { - /// println!("This better be printed as soon as possible!"); - /// }); - /// guard.flush(); - /// ``` - pub fn flush(&self) { - if let Some(local) = unsafe { self.local.as_ref() } { - local.flush(self); - } - } - - /// Unpins and then immediately re-pins the thread. - /// - /// This method is useful when you don't want delay the advancement of the global epoch by - /// holding an old epoch. For safety, you should not maintain any guard-based reference across - /// the call (the latter is enforced by `&mut self`). The thread will only be repinned if this - /// is the only active guard for the current thread. - /// - /// If this method is called from an [`unprotected`] guard, then the call will be just no-op. - /// - /// # Examples - /// - /// ``` - /// use crossbeam_epoch::{self as epoch, Atomic}; - /// use std::sync::atomic::Ordering::SeqCst; - /// - /// let a = Atomic::new(777); - /// let mut guard = epoch::pin(); - /// { - /// let p = a.load(SeqCst, &guard); - /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); - /// } - /// guard.repin(); - /// { - /// let p = a.load(SeqCst, &guard); - /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); - /// } - /// # unsafe { drop(a.into_owned()); } // avoid leak - /// ``` - pub fn repin(&mut self) { - if let Some(local) = unsafe { self.local.as_ref() } { - local.repin(); - } - } - - /// Temporarily unpins the thread, executes the given function and then re-pins the thread. - /// - /// This method is useful when you need to perform a long-running operation (e.g. sleeping) - /// and don't need to maintain any guard-based reference across the call (the latter is enforced - /// by `&mut self`). The thread will only be unpinned if this is the only active guard for the - /// current thread. - /// - /// If this method is called from an [`unprotected`] guard, then the passed function is called - /// directly without unpinning the thread. - /// - /// # Examples - /// - /// ``` - /// use crossbeam_epoch::{self as epoch, Atomic}; - /// use std::sync::atomic::Ordering::SeqCst; - /// use std::thread; - /// use std::time::Duration; - /// - /// let a = Atomic::new(777); - /// let mut guard = epoch::pin(); - /// { - /// let p = a.load(SeqCst, &guard); - /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); - /// } - /// guard.repin_after(|| thread::sleep(Duration::from_millis(50))); - /// { - /// let p = a.load(SeqCst, &guard); - /// assert_eq!(unsafe { p.as_ref() }, Some(&777)); - /// } - /// # unsafe { drop(a.into_owned()); } // avoid leak - /// ``` - pub fn repin_after<F, R>(&mut self, f: F) -> R - where - F: FnOnce() -> R, - { - // Ensure the Guard is re-pinned even if the function panics - struct ScopeGuard(*const Local); - impl Drop for ScopeGuard { - fn drop(&mut self) { - if let Some(local) = unsafe { self.0.as_ref() } { - mem::forget(local.pin()); - local.release_handle(); - } - } - } - - if let Some(local) = unsafe { self.local.as_ref() } { - // We need to acquire a handle here to ensure the Local doesn't - // disappear from under us. - local.acquire_handle(); - local.unpin(); - } - - let _guard = ScopeGuard(self.local); - - f() - } - - /// Returns the `Collector` associated with this guard. - /// - /// This method is useful when you need to ensure that all guards used with - /// a data structure come from the same collector. - /// - /// If this method is called from an [`unprotected`] guard, then `None` is returned. - /// - /// # Examples - /// - /// ``` - /// use crossbeam_epoch as epoch; - /// - /// let guard1 = epoch::pin(); - /// let guard2 = epoch::pin(); - /// assert!(guard1.collector() == guard2.collector()); - /// ``` - pub fn collector(&self) -> Option<&Collector> { - unsafe { self.local.as_ref().map(|local| local.collector()) } - } -} - -impl Drop for Guard { - #[inline] - fn drop(&mut self) { - if let Some(local) = unsafe { self.local.as_ref() } { - local.unpin(); - } - } -} - -impl fmt::Debug for Guard { - fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - f.pad("Guard { .. }") - } -} - -/// Returns a reference to a dummy guard that allows unprotected access to [`Atomic`]s. -/// -/// This guard should be used in special occasions only. Note that it doesn't actually keep any -/// thread pinned - it's just a fake guard that allows loading from [`Atomic`]s unsafely. -/// -/// Note that calling [`defer`] with a dummy guard will not defer the function - it will just -/// execute the function immediately. -/// -/// If necessary, it's possible to create more dummy guards by cloning: `unprotected().clone()`. -/// -/// # Safety -/// -/// Loading and dereferencing data from an [`Atomic`] using this guard is safe only if the -/// [`Atomic`] is not being concurrently modified by other threads. -/// -/// # Examples -/// -/// ``` -/// use crossbeam_epoch::{self as epoch, Atomic}; -/// use std::sync::atomic::Ordering::Relaxed; -/// -/// let a = Atomic::new(7); -/// -/// unsafe { -/// // Load `a` without pinning the current thread. -/// a.load(Relaxed, epoch::unprotected()); -/// -/// // It's possible to create more dummy guards. -/// let dummy = epoch::unprotected(); -/// -/// dummy.defer(move || { -/// println!("This gets executed immediately."); -/// }); -/// -/// // Dropping `dummy` doesn't affect the current thread - it's just a noop. -/// } -/// # unsafe { drop(a.into_owned()); } // avoid leak -/// ``` -/// -/// The most common use of this function is when constructing or destructing a data structure. -/// -/// For example, we can use a dummy guard in the destructor of a Treiber stack because at that -/// point no other thread could concurrently modify the [`Atomic`]s we are accessing. -/// -/// If we were to actually pin the current thread during destruction, that would just unnecessarily -/// delay garbage collection and incur some performance cost, so in cases like these `unprotected` -/// is very helpful. -/// -/// ``` -/// use crossbeam_epoch::{self as epoch, Atomic}; -/// use std::mem::ManuallyDrop; -/// use std::sync::atomic::Ordering::Relaxed; -/// -/// struct Stack<T> { -/// head: Atomic<Node<T>>, -/// } -/// -/// struct Node<T> { -/// data: ManuallyDrop<T>, -/// next: Atomic<Node<T>>, -/// } -/// -/// impl<T> Drop for Stack<T> { -/// fn drop(&mut self) { -/// unsafe { -/// // Unprotected load. -/// let mut node = self.head.load(Relaxed, epoch::unprotected()); -/// -/// while let Some(n) = node.as_ref() { -/// // Unprotected load. -/// let next = n.next.load(Relaxed, epoch::unprotected()); -/// -/// // Take ownership of the node, then drop its data and deallocate it. -/// let mut o = node.into_owned(); -/// ManuallyDrop::drop(&mut o.data); -/// drop(o); -/// -/// node = next; -/// } -/// } -/// } -/// } -/// ``` -/// -/// [`Atomic`]: super::Atomic -/// [`defer`]: Guard::defer -#[inline] -pub unsafe fn unprotected() -> &'static Guard { - // An unprotected guard is just a `Guard` with its field `local` set to null. - // We make a newtype over `Guard` because `Guard` isn't `Sync`, so can't be directly stored in - // a `static` - struct GuardWrapper(Guard); - unsafe impl Sync for GuardWrapper {} - static UNPROTECTED: GuardWrapper = GuardWrapper(Guard { - local: core::ptr::null(), - }); - &UNPROTECTED.0 -} |